Flip-chip bonding is a process by which a first semiconductor chip and a second semiconductor chip, both including fully-fabricated semiconductor devices, can be bonded together. According to the steps of the process, semiconductor chips are bonded together, and a flowble hardener, such as epoxy, is flowed between the bonded chips and allowed to harden. The substrate of the first chip can then be removed, if desired, such as to provide an unimpeded optical path to and from semiconductor optical components disposed thereon.
For substrate removal, acid, or other suitable etchant, is applied to the "back-side", i.e., the substrate-side, of the first chip. The etch removes the substrate but is prevented from attacking the semiconductor devices by a suitable stop-etch layer that is formed between the substrate and the device layers. The resulting structure has a plurality of semiconductor devices in a bed of hardener on the second chip. The mechanical integrity of the semiconductor devices is improved due to the surrounding hardener. Each semiconductor device is capped by a stop-etch layer. See U.S. Pat. Nos. 5,385,632 and 5,578,162.
Notwithstanding the benefits to the integrated structure, the hardener between the semiconductor devices may be subsequently removed in order to expose flip-chip bonding sites on the second chip so that additional chips may be bonded thereto. The hardener, if epoxy, can be removed by a dry etch. See, Ser. No. 08/766,214. Moreover, hardener removal may be necessary to access electrical probe pads or wire-bonding sites on the second chip. In particular, if several circuit designs, each requiring its own wire-bond pads, are aggregated onto a single die that is sectioned after processing, the hardener covering such wire-bond pads must be removed. Standard etch masks, such as photoresist, will not resist the hardener etch, so that such masks will not be able to protect underlying structure while the wire-bond pads are exposed. Thus, there is a need for a method to selectively remove the hardener underfill used when flip-chip bonding with substrate removal.